Project Abstract. Traumatic brain injury (TBI) is an extremely debilitating for the aging community as studies have found both increased incidence and severity within this population. Furthermore, TBI is the most predictive environmental factor for development of Alzheimer?s disease and other dementia related illnesses. The importance of age as a prognostic factor after TBI has long been recognized but limited studies has been devoted to understanding mechanisms that regulate secondary events that occur after the initial trauma. Even less research has been aimed at studying the mechanisms of cognitive loss in the elderly. The critical changes that affect cognition take place over a long period of time following the initial insult and the innate immune system activation is a key secondary injury mechanism that contributes to chronic neurodegeneration and loss of neurological function. In this proposal I will investigate the respective contribution of infiltrating macrophages and activated resident microglia in production of a neurotoxic and inflammatory milieu following TBI in an aging animal. Preliminary data for this proposal has found that TBI causes an exacerbated and prolonged CCR2+ macrophage infiltration in the aging brain when compared to young. The increased recruitment of peripherally derived monocytes significantly augments TBI-induced neuroinflammatory sequelae characterized by protracted activation of both microglia and macrophages as well as decreased synaptic integrity which may potentiate exacerbated injury-induced cognitive dysfunction observed in old animals. All together these findings demonstrate that, in the aging brain, peripherally derived macrophages have a distinct contribution to the TBI- related inflammatory response. In this proposal, I will identify the temporal relationships between macrophage infiltration, myeloid cell localization and inflammatory profiles in an aging animal after injury. Furthermore, I will investigate if blockade of macrophage infiltration can mitigate injury-induced neurotoxicity (dendritic spine loss and decreased synaptic integrity) thereby alleviating cognitive deficits. Findings from this work will advance mechanistic understanding of secondary mechanisms associated with TBI and test a pharmacological agent (already in clinical trials) for treatment of TBI-induced cognitive deficits in an aging animal.